In recent years the diesel engine industry has been under continuing pressure to reduce noxious emissions without unduly sacrificing fuel efficiency, or even while improving fuel efficiency. Engine emissions performance has improved, while maintaining acceptable fuel efficiencies, but pressure for further improvements remains.
An important element in these improvements is the modification of existing designs of diesel injection systems, particularly modification of existing injection nozzle-and-holder assemblies, especially the nozzles. In the never-ending pursuit of reduced exhaust emissions and improved fuel economy, modern fuel injection systems are operating at injection pressures considerably above those prevailing when ALCO injectors were introduced, and industry efforts are continuing to develop systems for still higher injection pressures. While it is not economically feasible to retrofit older engines with newer injection technologies, it is possible to make improvements in components of injection systems used with older engines and thereby increase to a meaningful extent the injection pressure at the nozzle orifices.
ALCO nozzle-and-holder assemblies and nozzles are a notable example of such systems. Similarly to some other older systems, those employing ALCO nozzles generally include a nozzle body, in which a nozzle body chamber is formed. The nozzle body terminates in a nozzle tip and houses a nozzle valve. The seat on which the nozzle valve closes is formed in the nozzle body at the bottom of the nozzle body chamber and is open-centered. It may be referred to as the body seat. Lower parts of the body seat lie in an imaginary conical surface. Below the nozzle body chamber is a small spray-hole feed chamber or “sac.” The spray holes, or orifices, are distributed around the sac and lead to the engine combustion chamber when the nozzle is installed.
One consideration in the design of such systems is the seat/orifice ratio, namely, the ratio, at full valve lift, between (i) the governing or minimum flow area at the body seat and (ii) the collective cross-sectional area of the spray holes. Lower seat/orifice ratios are associated with higher pressure drops through the body seat and lower injection pressures at the nozzle orifices, with a resultant degeneration of fuel penetration and fuel dispersion in the engine cylinder. Seat/orifice ratios over 2 or not too far below 2 are generally considered acceptable, while lower ratios are not. However, in certain high rated engines, when the orifice area required for the engine power rating gets to be too large for the nozzle size accommodated in the engine cylinder head, the seat/orifice ratio is considered not excessively restrictive down to 1.5, and in extreme cases is compromised down to 1.35.
In a rudimentary sense, the measure or value of the minimum flow area at the body seat depends on the sac diameter, since the minimum flow area at the body seat, when the valve is at full-lift position, is located adjacent the sac entry edge, where the side wall of the sac intersects the conical lower part of the body seat.
Increasing valve lift would of course increase minimum flow area at full lift, but there are well-known constraints on increasing lift, such as body seat impact damage and coordination of valve seating and engine stroke phases in high-rated engines.
Where good practice calls for increasing the seat/orifice ratio of an ALCO-type nozzle design without increasing valve lift, one way to do it is simply to enlarge the sac diameter, which has the effect of raising the altitude of the intersection between sac wall and body seat, thereby causing the unchanged spacing, at that raised altitude, between valve and body seat at full lift to sweep a greater circumference than at the lower altitude that previously applied, correspondingly increasing the minimum flow area at the body seat, thereby in turn increasing the seat/orifice ratio. It was recognized however, that such a modification of the ALCO nozzle would have a major disadvantage in that sac volume would be substantially increased by enlarging the sac diameter along the length of the sac, thereby tending to correspondingly degrade emissions performance.
In a case such as this when it is determined that the flow area through the seat is too small for the total nozzle orifice area, universal industry practice has been to reshape the sac in the region of its entry edge with a counter-boring tool having a 120° cutting edge bottom, so that the resulting counter-bore intersects the body seat at the raised altitude referred to above and forms an annular notch extending from the raised altitude referred to above to a level below the lower altitude referred to above—sufficiently below that there is little or no more restriction of flow at the bottom of the notch than at the top. While this modification has increased seat/orifice ratio while somewhat minimizing increase in sac volume, it has done nothing to reduce sac volume and improve emissions performance in that way. Moreover, even if sac volume had been reduced, as by foreshortening the sac, the configuration of the notch was such as to limit to some degree the effectiveness of such foreshortening in reducing emissions.
The present invention does contemplate reduction of sac volume by foreshortening of the sac. The present invention also involves annularly notching the body seat and sac wall to increase the seat/orifice ratio. However, according to the present invention, the notch is configured so that it detracts from the sac-volume-reducing effectiveness of the foreshortening of the sac to a much lesser degree than the above-described conventional type counter-bored notch would have if ALCO's sac had been foreshortened, or at least to a somewhat lesser degree, depending on the specific novel notch configuration selected.
The invention realizes these results by exploiting the geometrical fact that for solids generated by revolution of a polygon of given area (sweep area) around an axis in the same plane, relatively small percentage reductions of sweep area caused by trimming the radially outer side of the sweep area result in significantly larger percentage reductions of swept volume. This means that, in an injection nozzle, a relatively small percentage reduction in the sac's cross-section at its radially outermost parts results in a significantly greater percentage reduction in sac volume.
The improvements of the invention will be more fully understood from the following detailed description of the invention.